The invention relates to a pneumatically actuatable disc brake having a caliper straddling a brake disc. Brake pads can be applied to each side of the brake disc, of which an application-side brake pad is actuatable by an application device, preferably with a rotary lever of the application device. A brake cylinder, which can be pressurized with compressed air, is fastened to a flange of the brake caliper, which brake cylinder engages on the rotary lever via a cylinder plunger and has a return spring with which the cylinder plunger can be returned to an unloaded starting position The invention also relates to a brake cylinder which can be pressurized with compressed air.
A pneumatically actuatable disc brake of this type is known, for example, from EP 0 743 469 B1.
In this case, the brake cylinder is fastened directly to a flange of the caliper by use of bolts, the position of the brake cylinder on the brake, and therefore also the location of the compressed air connections, being determined by the bolts, whereby installation variants of the cylinder according to different installation conditions are produced.
In order to provide suitably high air pressure for actuating the rotary lever, corresponding dimensioning of the brake cylinder is required, resulting in a considerable number of disadvantages.
Because of the shocks and vibration that occur while the vehicle is being driven, the screw connections between the brake cylinder and the brake caliper must be of sturdy construction, as must a brake cylinder cover through which the fastening screws pass and against the inner wall of which the return spring also bears.
Moreover, the center of gravity of the brake cylinder is located relatively far from the connection region with the caliper, so that a correspondingly high bending moment is exerted on the fastening screws, requiring a design of the screws adapted accordingly.
Furthermore, the relatively great overall length of the brake cylinder determined by its function conflicts with the demands for optimization both of overall dimensions and of weight.
The plunger or piston of the brake cylinder passes through an opening present in the flange of the caliper, which opening is sealed from a secondary chamber of the brake cylinder by a bellows surrounding the plunger for this purpose.
A displacement volume corresponding to the stroke of the plunger is produced by compression or extension of the bellows, which displacement volume brings about pressure changes in the hermetically sealed caliper. Under heavy braking an overpressure of up to 0.4 bar can thereby be produced.
This problem can be solved according to the invention, as is considered in more detail hereinafter with reference to an example.
In such an overpressure phase, especially if a leak to the environment is present, air escapes to the outside, so that an under pressure is produced as the brake cylinder is retracted after braking.
A comparable effect is also produced by temperature fluctuations, to which a brake is heavily exposed by virtue of its function. In this case the air in the caliper expands upon heating while it contracts upon cooling.
Since the disc brakes preferably used in commercial vehicles are frequently operated in a weather-related moist or wet environment, if the aforementioned leak is present water is sucked into the interior of the brake in underpressure phases, which can lead, for example, to corrosion of mechanical components with the danger of complete failure of the brake.
The aforementioned long actuation strokes which must be executed give rise to a fragile sealing arrangement located far from the caliper, which is problematic with regard to handling and transportation of the disc brake parts before assembly, that is, before the brake cylinder has been installed, so that there is a risk of damage to the seal before installation of the brake cylinder on the brake caliper.
It is the object of the invention to develop a pneumatically actuatable disc brake and a brake cylinder such that they are optimized with regard to weight and installation space, that their service life is increased and that connections to the brake cylinder can be made in a simpler and more cost-effective manner (venting of position-dependent variant).
This and other objects are achieved by a disc brake, and associated brake cylinder, having a caliper straddling a brake disc. Brake pads can be applied to each side of the brake disc, of which an application-side brake pad is actuatable by an application device, preferably with a rotary lever of the application device. A brake cylinder, which can be pressurized with compressed air, is fastened to a flange of the brake caliper, which brake cylinder engages on the rotary lever via a cylinder plunger and has a return spring with which the cylinder plunger can be returned to an unloaded starting position. The return spring bears against the flange or against the base of a cover supported on the flange. The flange forms an abutment for the return spring
Advantageously, the flange forms an abutment for the return spring directly, or optionally via an interposed thin sheet metal covering foil which acts not only as an abutment.
Because of the small dimensions and low weight, the attachment of the brake cylinder to the caliper can therefore be considerably optimized, as compared to the previous embodiments. The cylinder may advantageously also form a preassembled unit without the cylinder plunger.
In addition, as a result of a self-energizing device, which has been used hitherto in electromechanically actuatable disc brakes, the power consumption of the brake cylinder is reduced, resulting in a substantially reduced space requirement with the same operating behaviour of the brake.
Even with relatively small self-energizing factors, a considerable reduction in the overall size of the brake cylinder and of a spring brake actuator is possible. In this case the self-energization is selected by suitable dimensioning of wedge or ramp angles in such a manner that reliably automatic release of the brake occurs even with a maximum possible coefficient of friction of the brake pads.
In contrast to electric-motor driven disc brakes, therefore, the power generator for applying the brake does not also need to be used for releasing the brake.
The reduction in overall dimensions is made possible, in particular, by the shorter stroke which, with a boost ratio of, for example, 2.7 and an adapted transmission ratio of the brake, is reduced from, for example, 65 mm hitherto to 24 mm.
Moreover, this short stroke has the result that the bellows surrounding the plunger can be configured to be very compact and, in the simplest case, is implemented with only one fold, the bellows with a steel-reinforced sealing seat being inserted directly into the through-opening at the interface of the brake cylinder with the caliper, and specifically into the opening of the caliper through which the plunger passes.
In principle, the plunger may be in two parts, consisting of a cylinder plunger, an end face of which rests against the brake lever, and a plunger plate which has an approximately cup-shaped depression into which the other end of the cylinder plunger is fitted. The plunger plate forms with a diaphragm the diaphragm piston which divides brake cylinder chambers formed, and which is pressurized with compressed air during braking. The cylinder may also be in the form of a pure diaphragm cylinder which does not have a spring brake actuator section.
Because of the small dimensions and low weight, the attachment of the brake cylinder to the caliper can be optimized in comparison to the previous configurations. Thus, the cover which closes the brake cylinder on its side oriented towards the flange may be implemented as a thin-walled sheet metal formed part, of the type of a largely dimensionally stable metal foil, against which one end of the return spring abuts while the other end bears against the plunger plate (diaphragm plate).
In this case the cover fits into the pot-shaped flange of the caliper which forms an abutment for the return spring, since the thickness of the sheet metal of the cover may be selected to be small enough that deformation-free abutment of the return spring against the cover would not be possible.
Preferably, the return spring is designed with the aim of bearing, directly or via a thin foil-like cover, against a flange of the disc brake as an abutment.
Minimization of the thickness of the cover also leads to weight reduction and to the elimination of fixing screws for fastening the brake cylinder to the caliper. Instead, the brake cylinder may be fastened to the caliper by way of a clamping strap, for example. For this purpose, firstly, the cover of the brake cylinder is configured as a rolled-in edge with which the diaphragm cylinder is retained on the brake cylinder housing. In this way, the brake cylinder is preassembled as a module with the brake cylinder housing, the diaphragm cylinder, the cover, the plunger plate and the return spring, while the cylinder plunger and the bellows are components of the preassembled caliper.
During assembly of the brake cylinder, the brake cylinder is simply placed on the pot-shaped flange of the caliper, the cylinder plunger centering itself practically automatically in the cup-shaped receptacle of the plunger plate.
A further advantage of the disc brake according to the invention will now be considered. With a conventional brake, the following is, for example, the case:                With a complete stroke of a known brake the displacement volume of the cylinder plunger in the bellows is 65 cm3.        With a stroke of 4.1 (i=15.6), the absorption volume of the extending plunger, optionally with bellows, is approximately 16.5 cm3.        The volume change ΔV is therefore:ΔV=65−16.5=48.5 cm3.        
With a self-energizing brake according to the invention, by contrast:                With a complete stroke of 30 cm the displacement volume of the cylinder plunger with bellows is approximately 30.5 cm3.        By contrast, with one of 4.16 cm (i=7.2) the absorption volume of the three extendable plungers with bellows is approximately 2.5 cm3.        Therefore:ΔV=30.5−25=5.5 cm3.        
The remaining volume change is insignificant and, with adapted dimensioning of the bellows, can be completely eliminated. Problems caused by underpressure or overpressure, as described in the introduction, are therefore avoided to a very large extent.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.